Portable telephone systems, such as second-generation digital cordless telephone (CT2) systems, typically include multiple call point stations each providing multiple radio frequency (RF) channels. These call point stations are also referred to as telepoints or cordless fixed parts (CFPs). CFPs allow persons using portable telephone handsets (also known as cordless portable parts, CPPs), to access the public switched telephone network (PSTN). Access to the PSTN can occur when a CPP gets in the range of a telepoint (CFP) and after the CPP (handset) has established a synchronous link with the CFP (base station).
In a CT2 system, a handset initiating a call to a base station asynchronously transmits on one available channel of the handset's transceiver, which corresponds to a radio frequency (RF) channel in the base station (each base station being capable of supporting up to 40 channels).
In a typical CT2 system, the communication protocol standard includes four main burst structures, called multiplex 3 (MUX 3), multiplex 2 (MUX 2) and multiplex I (MUX 1) which is further subdivided into either multiplex 1.4 or 1.2 (MUX 1.4 or MUX 1.2). MUX 3 is utilized mainly for communication link initiation (link establishment and re-establishment) from a CPP to a CFP. MUX 2 is used primarily for communication link establishment and for link initiation from the base station(CFP). The MUX 1 burst structures (MUX 1.2 and MUX 1.4) are used primarily for voice/data communications, signaling information, and control messages from the CPP (portable) and CFP (base). For a better understanding of the overall CT2 system communication protocol, one is referred to a publication entitled, "Common air interface specification to be used for the interworking between cordless telephone apparatus in the frequency band 864.1 MHz to 868.1 MHz, including public access services", Version 1.1, dated 30th June 1991, which is published by the European Telecommunications Standards Institute and is hereby incorporated by reference.
In FIG. 1, a table 100 showing the typical handshake sequences between a cordless portable part (CPP) and the telepoint base station (CFP) is shown. Table 100 shows the link setup direction, the message direction, MUX mode, and the content of the link identification code (LID) for each handshake sequence. The LID code is used for the following:
i). End point identification for CPP (handset) call setup; PA1 ii). Link reference for associating CPP (handset) and CFP (base) calls during handshake exchanges and link re-establishments; and for iii). Base identifier (BID), which is the ringing address to which one or more CPP's will respond to.
Row 102 shows a basic sequence on how a handset (CPP) establishes communications with a telepoint (CFP), row 104 shows how a telepoint station establishes communications with a handset, and row 106 shows how a communication link is re-established in case the link is disrupted during an ongoing communication session. Looking for example at row 102, the sequence of events which occur in order to establish a communication link between a particular handset (CPP) and base station (CFP), is shown. The requesting handset (CPP) transmits in MUX 3 a message including the identification number of the handset (end point ID). This is followed by the base station transmitting in MUX 2 a message with the LID having the communication link reference identification number which informs the handset that the link has been granted.
Once the communication link establishment has occurred between the two units, the units go to the MUX 2 protocol where the CFP transmits a supervisory message and acknowledgment. The capabilities of both the CPP and CFP are determined, and authorization to use the system is determined by the CFP. The two units then move on to the MUX 1 protocol by a request from the CFP and an acknowledgment by the CPP. Once in MUX 1, dial tone is sent out by the CFP, the CPP then dials the telephone number he wishes to access. Finally, this is followed by a voice communication link being established between the CPP and the party at the dialed number telephone number. The voice communications are either transmitted using MUX 1.2 or MUX 1.4 format depending on the specific system 100 being utilized.
In FIG. 2, a simplified MUX 3 link initiation packet is shown. The two other important communication protocols utilized in a CT-2 communication system are MUX 2, shown in FIG. 3, and MUX 1 (implemented either via MUX 1.2, 2 bit signaling, or MUX 1.4, 4 bit signaling), shown in FIG. 4. MUX 2 is used after link initiation under MUX 3 has been achieved. MUX 2 is used to carry the D, or signaling channel information, and SYN, or synchronization channel information, for communication link establishment and re-establishment. B channel information which carries the 32 kbit/second speech or data is nonexistent in MUX 2. MUX 2 carries the D channel at a data rate of 16 kbit/second and the SYN channel at an overall rate of 17 kbit/second. The SYN channel consists of ten bits of preamble comprised of one and zero transitions followed by a channel marker (CHMF), or synchronization marker (SYNC). Data bits in the D channel are aligned in MUX 2 so that the D channel synchronization word, SYNCD, occurs as the first 16 bits in the D channel after the SYN channel.
In FIG. 5, a prior art MUX 2 synchronization pattern is shown. When data is sent into the SYNC detector circuit of a communication device such as a CT2 cordless handset, and the data being received correlates to the synchronization pattern expected, the receive bit counter within the receiver is re-aligned to the correct position, and frame synchronization in MUX 2 is accomplished.
Referring to FIG. 6, a prior art MUX 1 communication packet highlighting the inversion bits is shown. In MUX 1 there are 2 D channel (data) bits and 64 B channel (voice bits). The SYN channel which is found in MUX 2 and 3 is not found in MUX 1. MUX 1 supports both the 68 bit and 66 bit burst structures (referred to as MUX 1.4 and MUX 1.2 for signaling with four data bits 602 or two data bits 604 respectively). In MUX 1.4 the valid transmitted data bits in a burst are numbered 1 to 68, and in MUX 1.2 they are numbered 2 to 67. In a CT2 system it is required to invert 32 of the 64 bits as they are transmitted across the communication link in order to ensure reasonably random bit sequences. In MUX 1.2 and MUX 1.4, the following bits as shown in FIG. 6 are inverted: 03, 04, 06, 09, 14, 16, 18, 19, 20, 22, 23, 27, 28, 29, 30, 31, 34, 35, 37, 40, 45, 47, 49, 50, 51, 53, 54, 58, 59, 60, 61, 62. These bit numbers refer to the 64 bits of the B channel and not the bit numbers of the data burst. Bit 1 is the first transmitted bit of the B channel. When voice is active in the communication link, the B channel bits are randomly changing based on the encoded adaptive differential pulse code modulated (ADPCM) data.
The MUX 1 signaling packet shown in FIG. 4, is invoked from MUX 2 by a control message. MUX 1 is used bi-directionally over an established link to carry D channel (Data) and B channel (speech/data). The SYN (synchronization) channel is nonexistent in this multiplex and therefore should burst synchronization be lost, it cannot be recovered without re-initializing the communication link. MUX 1 supports both 68 bit (referred to as MUX 1.4) or 66 bit (referred to as MUX 1.2) burst structures. It is up to each system manufacturer to decide which of the two to utilize for MUX 1 signaling. MUX 1 data rates are 2.0 kbit/second for MUX 1.4 and 1.0 kbit/second for MUX 1.2. Data bytes in the D (data) channel are aligned in this multiplex so that bytes always start on a frame boundary. In order to ensure random bit sequences in MUX 1, a certain known number of the B channel bits are inversed in each frame.
If the communication link between a cordless telephone and a CT-2 base station is lost (e.g., due to multipath fading, radio frequency interference, etc.) after the link is already in MUX 1 (voice communication has begun), the units (i.e., the base station and handset) must both return to the MUX 3 or to the MUX 2 protocol level in order to re-synchronize with each other (also known as link re-establishment) since resynchronization is not available in MUX 1. This process of moving down to MUX 3 then to MUX 2 and then back to MUX 1 is very time consuming, typically taking over 1 second to accomplish. This causes the handset user to experience a 1 second or greater time frame in which their is no communications. A need thus exists in the art for a method and apparatus which can accomplish synchronization when a communication link is lost in a communication system such as a digital cordless telephone system in a more efficient manner.